Before now, various types of injection molding methods and apparatuses have been proposed as injection molding methods and apparatuses to attain comparatively precise molded products from synthetic resin, such as plastic.
In particular, in recent years, an injection molding method and apparatus of valve gate type, has been proposed as one of the injection molding methods and apparatuses, in order to reduce the molding cycle time for a molded product.
This valve gate-type injection molding method and apparatus comprises a valve for temporarily holding molten resin to which prepressure is applied, and a valve pin extendably and retractably placed within the valve, wherein by releasing a gate formed in the mold by the valve pin, molten resin, to which prepressure was applied, fills the cavity in the mold at a high speed, and the gate is closed again by the valve pin after the filling of the molten resin.
This type of valve gate-type injection molding method and apparatus has the effect of eliminating the time wasted by the compression of the molten resin before its injection, and therefore shortening the molding cycle time for a molded product, because molten resin within the valve is compressed in advance during the opening of the mold to take out the molded product.
In the case of molding a molded product having a hollow portion with the conventional valve gate-type injection molding method and apparatus discussed above, a large number of mold parts having complex forms are prepared in the portion of the mold for forming the hollow portion, in view of releasing that portion of the mold, and an operation of removing the large number of mold parts having complex forms in a proper order is performed when removing a molded product having a hollow portion from the mold.
For this reason, in the case of molding a molded product having a hollow portion with the conventional valve gate-type injection molding method and apparatus discussed above, disadvantages are that the parts of the mold are complex and great in number, and therefore costs for producing the entire valve gate-type injection molding apparatus is increased, and also efficiency for producing hollow molded products is lowered due to the complex releasing operation for the hollow portion.
Moreover, in the case of molding a hollow product, such as a bottle with a small mouth, it is impossible to release the portion of the mold for forming the hollow portion. Therefore, conventional valve gate-type injection molding methods and apparatuses cannot be used to form a hollow molded product of such a type as a bottle with a small mouth.
With the foregoing in view, it is an object of the present invention to provide a valve gate-type injection molding method and apparatus capable of improving as much as possible the production efficiency for a hollow molded product, while reducing the number of parts of the entire apparatus and the number of manufacturing processes.